1. Field of the Invention
The present invention relates to a novel metal oxide material which exhibits superconductivity. In particular, the present invention relates to a metal oxide material that is useful in various application fields of superconductivity such as sensors, electronic devices, computers, medical apparatuses, magnets, power transmission lines, energy instruments, and voltage standards. The metal oxide material of the present invention is especially effective when used as a bulk material. The metal oxide material of the present invention can be used in a state of junction or dispersion with another oxide or a metal.
2. Related Background Art
Copper-containing oxide superconductors which have been discovered in succession have critical temperatures of superconductivity (Tc) surpassing that of known metal-type superconductors such as niobium compounds. Therefore, applications thereof are intended in various fields. The known copper-containing oxide superconductors include those of Bi-system, Tl-system, Pb-system, Y-system, and La-system.
Typical of the copper-containing oxide superconductors, those composed of Sr, Ln (Y or a lanthanoid element), Cu and oxygen having a composition of YSr.sub.2 Cu.sub.3 O.sub.y are disclosed in papers of Japanese Journal of Applied Physics, Vol. 26, L804 (1987); Solid State Communication, Vol. 63,535 (1987); and Preprint for Autumnal Meeting of Japan Physical Society, Third Part, page 243, 2p-PS-30!. Further, another superconductor having the composition YSr.sub.2 Cu.sub.3-x MO.sub.y (M=Al, Fe, Co, or Pb, and 0.4 .ltoreq.x.ltoreq.1.0) is disclosed in Chemistry of Materials, Vol. 1, 331 (1989).
The aforementioned Bi-system, Tl-system, and Pb-system of superconductors undesirably have a specific gravity of as high as from 7 to 8 g/cm.sup.3 so that a bulk material (for example, a shield material) made therefrom inevitably becomes disadvantageously weighty in its entirety. A known Sr-based 123-phase material, which has a lower specific gravity than the above-mentioned material, has disadvantages of unavoidable contamination by a large amount of impurities, difficulty in the synthesis, and the low Tc (approximately 20K).
For example, the compositions disclosed in Japanese Journal of Applied Physics, Vol. 26, L804 (1987) and Solid State Communication, Vol. 63,535 (1987) do not give a single phase high-quality sample of YSr.sub.2 Cu.sub.3 O.sub.y, but form many impurities such as SrCuO.sub.2, Sr.sub.2 CuO.sub.3, Y.sub.2 SrO.sub.4, Y.sub.2 CuO.sub.5, SrCu.sub.2 O.sub.2, and Sr.sub.1.75 Cu.sub.3 O.sub.5.13, thus the product is not practically useful. Thus the synthesis products of the Sr-based 123-phase material contain the superconducting material only in a small amount even though the product is satisfactory in its light weight, and are therefore, not useful.
The sample described in the aforementioned Preprint for Autumnal Meeting of Japan Physical society, Third Part, page 243 was synthesized by means of a special apparatus employing conditions of 70 Kbar and 1380.degree. C. which was not generally available, therefore being unsuitable for practical applications. Furthermore, even with such a special apparatus, the synthesized product exhibited a zero-resistance temperature (the temperature at which the resistance becomes zero) of about 20K.
The material disclosed in Chemistry of Materials, Vol. 1,331 (1989), which exhibits superconductivity in the case of M=Co or Fe, and has the zero-resistance temperature of as low as 10K, has a superconductivity volume fraction of about 2%, thus being unsuitable for a superconducting material.